This invention relates to a thermal transfer image-receiving sheet which, in use, is superposed onto a thermal dye donor sheet, and more particularly to a thermal transfer image-receiving sheet having texture similar to plain paper.
Various thermal transfer recording systems are known in the art. Among these known systems is a thermal dye transfer system, wherein a sublimable dye as a colorant is transferred, using a thermal head capable of generating heat in response to a recording information, onto an image-receiving sheet to produce an image.
According to this recording system, since a sublimable dye is used as a colorant, density gradation can be controlled as desired and can reproduce a full-color image of an original image. Further, the formed dye image is very sharp and highly transparent and hence is excellent in reproduction of halftone and gradation, realizing a high-quality image comparable to a silver-salt photographic image.
A plastic sheet, a laminated sheet composed of a plastic sheet and paper or the like, or a synthetic paper or the like has been used as a thermal transfer image-receiving sheet in the thermal dye transfer system. In order to spread utilization of the thermal dye transfer system to general offices, use of plain papers, such as coated paper (art paper), cast coated paper, and paper for PPC, as a substrate sheet for the image-receiving sheet has been proposed in the art.
The conventional thermal transfer image-receiving sheet comprising a dye-receptive layer provided on one side of a paper substrate often poses the following problems. Specifically, paper per se contains water in an amount of about several % by weight. The amount of water contained in terms of % by weight refers to water content. The water content is not always constant and varies depending upon environmental humidity. Under high humidity environment, the paper substrate absorbs moisture in the air, leading to an increase in dimension of pulp which is a main constituent of the paper. On the other hand, under low humidity environment, the paper substrate releases water contained therein into the environment, leading to a reduction in dimension of the pulp. The dimensional change is significant in the cross direction of fibers of the pulp rather than in the direction of fibers in the pulp. The dimensional change of the pulp results in the dimensional change of the paper per se.
In a paper substrate which has been continuously produced by a conventional continuous papermaking machine, the pulp is likely to align in the machine direction (MD) of paper. Therefore, the dimensional change created by the absorption and release of moisture is more significant in the cross direction (CD) relative to the machine direction. Thus, the thermal transfer image-receiving sheet using a paper substrate creates a dimensional change due to absorption and release of moisture, often causing troubles associated with loading of the thermal transfer image-receiving sheet into a printer or carrying of the thermal transfer image-receiving sheet within the printer.
Further, a thermal transfer image-receiving sheet comprising a paper substrate and at least a dye-receptive layer provided on one side of the paper substrate often causes curling due to a difference in stretching behavior between the substrate portion and the portion of several layers including the dye-receptive layer. The curling leads to troubles associated with loading of the thermal transfer image-receiving sheet into a printer or carrying of the thermal transfer image-receiving sheet within the printer and in addition remarkably deteriorates the appearance of the print.
Further, in the thermal transfer method, at the time of printing, a thermal transfer sheet is heated by means of a thermal head or the like to transfer a colorant onto a thermal transfer image-receiving sheet in intimate contact with the thermal transfer sheet. Therefore, the temperature of the thermal transfer image-receiving sheet also is increased. This causes water contained in the substrate to be evaporated to create a dimensional change.
In particular, in printing a color image, an original image is subjected to color separation into three colors of yellow, magenta, and cyan, or four colors of yellow, magenta, cyan, and black, and colorants of respective colors are successively transferred to form a color image. Therefore, in transferring each color, the image-receiving sheet gradually undergoes a dimensional change, often leading to misregistration of image.
Further, when the water content of the image-receiving sheet before printing, that is, the image-receiving sheet at the time of preparation thereof, is lower than a given value, moisture absorption occurs at the time of paper feed. Also in this case, the image-receiving sheet undergoes a dimensional change, often leading to misregistration of image.
In the case of an image-receiving sheet in a sheet form, when several sheets are put on top of another on a feeding tray, the interior sheet located between surface sheets does not easily undergo a change in water content. Since, however, at the time of feeding, both sides of the image-receiving sheet are exposed to printing environment, a change in water content, that is, a dimensional change, often occurs. On the other hand, in the case of an image-receiving sheet in a roll form, the center portion of the roll does not easily undergo a change in water content. In this case as well, in feeding, both sides of the image-receiving sheet are exposed to printing environment, often leading to a change in water content, that is, a dimensional change.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer image-receiving sheet that causes none of a dimensional change, curling, and misregistration of image and can produce a printed image having satisfactory image quality and density.